Fatigue response up to 10^9 cycles of a structural epoxy adhesive

In the present paper, the very-high-cycle fatigue (VHCF) response of a structural adhesive used for automotive applications, Betaforce 4600G modified with microspheres, has been experimentally assessed. Ultrasonic fully reversed tension–compression tests up to 10^9 cycles have been carried out with the testing machine developed by the authors on adhesives without macroscopic defects and on adhesives with artificial defects, inserted during the butt-joint preparation. Fracture surfaces have been observed with the optical microscope and the P-S-N curves estimated. Experimental results have shown that defect location significantly affects the VHCF strength and fracture surfaces exhibit a peculiar morphology with three distinct characteristic regions